1. Field of the Invention
The invention concerns a regulating device for a burner and includes an ionization electrode arranged in the flame region of the burner, and a setting member which influences the fuel flow or the air flow depending on a setting signal.
2. Description of the Prior Art
Ionization electrodes for flame monitoring purposes in burners have long been in use. In general, however, the ratio of the amount of air to the amount of fuel, often referred to as lambda, is mutually matched in regard to any output demand either by a control system (i.e. without feed-back or the like) or by a regulating system (i.e. with feed-back or the like) and sensors. In general lambda at each output demand should be slightly above the stoichiometric value of 1, for example 1.3.
Unlike controlled burners, air ratio-regulated burners react to external influences which alter combustion. They therefore have a higher degree of effectiveness and thus a higher level of efficiency as well as lower pollutant emissions and thus a lower level of environmental pollution. The sensors required for that purpose, often gas sensors, in particular oxygen sensors, or temperature sensors, are however expensive, unreliable, require maintenance and/or involve a short operating life.
For that reason, for many years now burner manufacturers and regulating device manufacturers have endeavored to use the ionization electrode which is already present, not just for flame monitoring but also as a sensor for burner regulation purposes. German Patent DE-A1-39 37 290 describes a test structure for regulating the gas-air ratio, in which the ionization electrode is supplied with a dc voltage. That principle is not very suited to mass production. Monitoring the flame with the same ionization electrode is not possible as for that purpose only the rectifier property of the flame may be used.
Italian Patent IT-95U000566 and European Patent EP-A1-909 922 which describe regulating devices for gas burners appeared some years ago. In a simplified illustration, those specifications describe how the setting member is controlled on the basis of a stored characteristic in the event of dynamically rapid changes in the gas or air volume flow. In contrast, in the event of slow changes in the gas or air volume flow, a fine setting occurs on the basis of a regulation with the ionization signal as a measurement parameter.
Rapid changes in the fuel flow or the air flow typically occur due to sudden changes in the output demand. In addition, changes in air ratio and thus changes in the fuel or air volume flow can be caused by a change in the fuel composition, a change in the air pressure, changes in the gas pressure, temperature changes, fouling and wear of mechanical burner components, and so forth.
The stored characteristic in the regulating devices disclosed in the Italian Patent IT-95U000566 and the European Patent EP-A1-909 922 establishes at each air pressure of the blower, and thus at each demanded level of power, a setting signal which corresponds to an approximately desired status of the setting member for the gas valve. Also described is an alternative regulating device in accordance with which the air volume flow is adapted to the gas volume flow and the stored characteristic approximately establishes the desired blower speed depending on the setting parameter of the gas valve.
A burner-specific characteristic is obtained by the burner being operated under a respective different loading with changing setting member statuses, in which case emission values and a level of efficiency are measured with additional sensors and the desired setting parameters are determined in that way.
Air ratio-regulated burners have advantages over apparatuses which are controlled by means of characteristics. With a constant output, changes in temperature, fuel pressure, air pressure, fuel composition, wear and fouling of mechanical components etc. allow the set working point to drift away.
For that reason, the regulating devices disclosed in the Italian Patent IT-95U000566 and the European Patent EP-A1-909 922 admittedly implement control on the basis of the stored characteristic when rapid changes in output occur, but compensate for the incompleteness thereof insofar as they initially displace the last status of the setting signal a constant distance along the stored characteristic to a new value.
Approximately at the same time the proprietor of the European Patent EP-A2-806 601 developed regulating devices which also have a characteristic stored for the setting signal. The characteristic also basically serves to provide for preliminary control of the setting member in the event of rapid changes in output.
The last-mentioned regulating devices include an ionization evaluating device which is connected downstream of the ionization electrode and which produces an ionization signal, a control unit in which characteristic data for determining a first mode of behavior of the setting member are stored, which at least at times produces a first control signal, and a regulator which produces the above-mentioned setting signal at least at times depending on the ionization signal and at least at times depending on the first control signal.
The above-indicated regulating devices known from the state of the art have disadvantages. More specifically, some use additional sensors and/or do not hold the air-gas ratio very stable upon dynamic variations in output. Market acceptance is correspondingly low.
According to a first aspect of the present invention, there is provided a regulating device for a burner
comprising an ionization electrode arranged in a flame region of the burner, and
a setting member which influences a fuel flow or an air flow depending on a setting signal, and is
equipped with an ionization evaluating device which is connected downstream of the ionization electrode and which produces an ionization signal;
a control unit in which characteristic data for determining a first mode of behavior of the setting member are stored and which at least at times produces a first control signal;
a regulator which produces the setting signal at least at times depending on the ionization signal and at least at times depending on the first control signal; and
wherein also stored in the control unit are characteristic data for determining a second mode of behavior of the setting member,
the control unit produces at least at times a second control signal, and
the regulator produces the setting signal at least at times depending on the second control signal.
It has been found that a substantial improvement in terms of regulating a burner by way of the ionization electrode lies in the features of the invention that characteristic data for determining a second mode of behavior of the setting member are stored in the control unit, the control unit at least at times produces a second control signal, and the regulator produces the setting signal at least at times depending on the second control signal.
The structure of a regulating device according to the invention requires few resources such as electronic components and computing capacity of a microprocessor. For a one-off initial setting of a regulating device to a certain type of burner, instead of previously one, now two or more burner-specific characteristics have to be established.
Practice has shown that the second control signal makes an above-average contribution to making control of the setting signal more precise.
The regulating device moreover can be so designed that, upon the detection of suitable conditions, it itself implements a setting method for the detection of new characteristic data.
Thus, occasional or regular re-calibration takes place, in order to compensate for any incremental changes in the regulating system, for example wear or fouling of the ionization electrode. Another possible option provides that the control characteristics are determined automatically, even for gases having characteristics that are not covered by the pre-set characteristics.
The characteristic data can be, for example, in the form of the constants in a polynomial up to the third order. The function that is approximately represented by the polynomial establishes a relationship between an input parameter and the setting signal.
The input parameter used for the control curves is primarily the output demand, either in the form of a setting parameter or a measured parameter that corresponds to the output, for example, the speed of rotation of the blower. It will be appreciated that it is also possible to use other values or parameters as an input value for the control characteristics, for example temperature signals of all kinds such as burner temperature, flow and return temperature, and so forth. Further examples are a pressure difference measurement value for determining the gas or air volume flow, a gas or air volume flow measuring device or directly the actuating signal for operation of a gas valve or an oil pump.
Advantageously, the first and second modes of behavior of the setting member depend on input parameters which represent the same value. The measured output demand, or another physical value, can be fed to the control unit by means of a single input parameter, such as the setting value with respect to the speed of rotation of the blower, or by means of input parameters of a different nature, such as the setting parameter and the measured parameter with respect to the blower speed.
The above, however, is not necessarily so. If, in particular, the regulating device has further measurement values available during operation from which it can directly or indirectly determine, for example, the current energy content or the current pressure of the fuel being supplied, then the second input parameter can represent a different value.
Burners are often equipped with a temperature sensor for the boiler temperature. A change in the energy content of the fuel supplied results in a change in the boiler temperature. In the case of such a burner, for example, the setting parameter for the blower speed is the first input parameter, and the change with respect to time of the boiler temperature is the second one. Characteristic data have been stored that determine a first desired mode of behavior of the setting member at different outputs, but with a fixed energy content of the fuel and with other influences also fixed. Also, characteristic data have been stored that determine a second mode of behavior with different energy contents and at a fixed output.
In the previous example, the regulating device, on the basis of changes in boiler temperature which do not correspond to the variation with respect to time of the setting parameter for the blower speed, determines any changes in the current energy content of the fuel being supplied and by use of the characteristic data for the second mode of behavior and the ionization signal, produces a corrected output-dependent control curve. In the case of a dynamic change in output the setting signal will follow the control curve corrected in that way, for example, at a distance which remains uniform.
A wide variety of burner types can be considered here, for example pre-mix gas burners or atmospheric burners with or without an auxiliary blower. In the case of atmospheric burners without an auxiliary blower the air volume flow can be controlled, for example, by way of an air flap or the like.
In an advantageous embodiment of the invention the regulator produces the setting signal at least in part by processing the control signals and determines the mode of processing at least at times depending on the ionization signal.
This invention includes a number of variants. For example, the control unit produces no control signals in a quasi-stable state. The regulating device then implements regulation by way of the ionization signal. As soon as a rapid change in state occurs, however, the regulating device switches over to the rapidly reacting and accurate control by virtue of processing the control signals. The way in which the control signals are processed is previously established, for example, by the ionization signal and remains the same throughout the entire control period. Control is only replaced by regulation again when the state has settled down and the ionization signal trails the current state. In accordance with an alternative embodiment, however, the control signals are permanently produced and both the control signals and also the ionization signal contribute continuously to the setting signal. Hybrid variants are also possible.
In particular it has proven to be advantageous that the regulator at least at times weighs and adds the control signals and that the regulator at least at times determines the weighing depending on the ionization signal.
In an advantageous embodiment of the invention the regulator damps rapid fluctuations in the ionization signal in comparison with slow fluctuations prior to processing the control signals. In particular, the regulator is equipped with a low pass filter for the ionization signal or for a signal resulting therefrom by processing, or with an integrating unit for the ionization signal or for a signal resulting therefrom by processing.
These measures initially only adjust the mode of processing the control signals with a certain delay and/or a smoothing of the ionization signal so that the ionization signal variation that in any case is too sluggish, after a sudden change in state, does not disturb the setting signal. It is only when the situation has settled down again that the ionization signal will slowly act on the mode of processing the control signals in order to allow fine tuning.
In a further embodiment of the invention the control unit also stores characteristic data for determining a mode of behavior of the ionization signal, the control unit produces at least at times a target value signal for the ionization signal, and the regulator produces the setting signal at least at times depending on the target value signal.
By virtue of those measures, the regulating device, or the regulator program thereof, can be of a simple configuration and achieve a high level of reliability. Optionally, the regulating device occasionally or regularly calibrates characteristic data.
In one embodiment of the invention the regulator is advantageously equipped with a comparison unit which at least at times subtracts from the ionization signal the target value signal or a signal resulting therefrom by processing. In this embodiment the regulator can so produce the setting signal that the ionization signal is regulated to the target value signal. That difference can be regulated to zero by means of the above-mentioned integrating unit.
A further embodiment of the invention concerns the stored characteristic data. Advantageously, the first mode of behavior of the setting member has been determined during a burner operation with a first fuel and the second mode of behavior of the setting member has been determined during a burner operation with a second fuel which is different in terms of energy content, in particular if the specific energy content of a fuel is at least 5% higher than that of another fuel.
It has been found that the characteristics, as from that limit value, are so different from each other that they give the regulating device substantial additional information as compared with a regulating device with only one stored characteristic. This substantially increases the extent of some advantages of the invention.
In another embodiment the characteristic data for determining the two modes of behavior of the setting member result from measurements. Alternatively, however, only the characteristic data for the first mode of behavior of the setting member are determined on the basis of measured results. The characteristic data for the second mode of behavior are then calculated from those measured results.
In a variant of the above-indicated embodiment the characteristic data for the second mode of behavior are established on the basis of knowledge of the man skilled in the art about fuel mixtures which are supplied in practice, instead of by means of burner-specific measurements.
Therefore, setting of a regulating device to a certain type of burner is advantageously implemented by two or more burner-specific characteristics being established during operation with different fuels, for example gas mixtures in different conditions.
The invention also includes a method of setting a regulating device. In accordance with that method a burner is equipped with a regulating device according to the invention and with additional sensors for establishing the quality of combustion. Then, the burner is operated with a first fuel with a certain energy content at different output values with respectively different setting member statuses, in which case a desired setting member status is established from the sensor results for each output value. Characteristic data for determining the first mode of behavior of the setting member are established from the desired setting member statuses. Thereafter, the burner is operated with a second fuel with a different energy content at different output values with respectively different setting member statuses, in which case a desired setting member status is established from the sensor results for each output value, and characteristic data for determining the second mode of behavior of the setting member are established from the desired setting member statuses. These steps are optionally repeated for a third or even further fuels. Finally, the established characteristic data are stored in one or more regulating devices. As described above, advantages are entailed in the specific energy content of a fuel being at least 5% higher than that of another fuel.
Alternatively, the burner is operated with a fuel flow under a first pressure at different output values with respective different setting member statuses, in which case a desired setting member status is established from the sensor results for each output value. Characteristic data for determining the first mode of behavior of the setting member are established from the desired setting member statuses. Thereafter the burner is operated with a fuel flow under a different second pressure at different output values with respective different setting member statuses, with a desired setting member status being established from the sensor results for each output value. Characteristic data for determining the second mode of behavior of the setting member are established from the desired setting member statuses and the established characteristic data are stored in the regulating device.